In so-called pressure diffusers used in the cellulose pulp industry for washing pulp in continuously operating washing columns, the screen unit is given a reciprocating movement with the aid of a hydraulic working cylinder via a drag bar. The stroke length is normally up to about one meter. During the downward working stroke, the screen unit is fed slowly downwards at a speed which only slightly exceeds the speed at which the cellulose pulp falls through the column. By contrast, the speed on the return stroke is high: normally about 1-1.5 m/sec. The screen unit which is to be lifted during this rapid return movement can weigh over ten tones, and even bigger installations have been planned. In addition to this, there is the friction between the screen unit and the cellulose pulp in the column. The working cylinder and other parts of the hydraulic unit must therefore be given very large dimensions so as to be able, within a short time, to perform considerable work. This entails, for example, hydraulic oil flow rates of over 10,000 l/min; oil quantities which additionally have to be filtered and cooled during the work cycle of the hydraulic unit. The systems existing at present do not solve these problems in a satisfactory manner, a fact which poses an obstacle to developments within this area of technology towards ever bigger pressure diffusers and, thus, ever heavier screen units.
It is an object of the invention to remedy the above problems and offer a solution aimed at lightening the hydraulic system. It will be appreciated, however, that although the invention has been developed with the aim of solving the problems which are acute in the field of pressure diffusers within the cellulose industry, it is also possible to envisage the invention having another area of application.
These and other objects of the invention can be achieved by the fact that it is characterized by what is stated in the attached patent claims. Further characteristics and aspects of the invention will be evident from the following description of a preferred embodiment.